1. Field of the Invention
The present invention relates to a gap control device for a Pilger die assembly of cold Pilger mills, in general, to a gap control device which can independently control the height of a pair of bearing blocks which axially support an upper die.
2. Description of the Related Art
Cladding pipes of a nuclear fuel assembly serve to separate UO2 pellets from coolant in the core, prevent a radiant fission product produced from the UO2 pellets due to being discharged into the coolant, and prevent a chemical reaction between the coolant and the UO2 pellets by separating the coolant and the UO2 pellets from each other. Cladding pipes are made of a zircaloy or zirconium alloy that has superior corrosion resistance to the hot coolant and low neutron absorptivity.
Korean Laid-Open Patent Publication No. 10-1986-0005894 (dated Aug. 16, 1986) or Korean Laid-Open Patent Publication No. 10-2000-0005310 (dated Jan. 25, 2000) disclosed a process of fabricating cladding pipes. The process includes manufacturing an ingot by adding several alloy elements; manufacturing a pipe reduced extrusion (TREX) from the ingot by hot extrusion; and reducing the thickness and diameter of the TREX by repeating cold processing, referred to as Pilgering, and heat treatment processing, whereby a cladding pipe made of a Zr alloy is finally fabricated.
FIG. 1 is a configuration view showing a typical Pilgering apparatus for a cold milling process. The typical Pilgering apparatus includes a pair of rotatable Pilger dies 10 and a mandrel 20. The Pilger dies 10 transport a roll stand (or a saddle) 30 back and forth within a certain stroke range.
The mandrel 20 is inserted into a pipe 1 having a greater diameter, the pipe 1 being made of a Zr alloy. While the pipe 1 is being rotated and transported between the pair of Pilger dies 10, the inner diameter, the outer diameter, and the thickness of the pipe 1 are reduced by the Pilger dies 10 and the mandrel 20, whereby the pipe is fabricated into a pipe having certain dimensions through extrusion.
FIG. 2 is a side elevation view showing the typical Pilgering apparatus. The Pilger dies 11 and 12 consisting of the upper die 11 and the lower die 12 are rotatably assembled to the roll stand 30. The pipe is inserted in the working direction D between the upper and lower dies 11 and 12.
The upper die 11 is movable upwards and downwards perpendicularly to the working direction D, and a gap control device 40 for controlling a gap G between the upper and lower dies 11 and 12 is provided. Specifically, the gap control device 40 includes a first adjustment wedge 41 disposed on the upper die 11, a second adjustment wedge 42 which is in surface contact with the first adjustment wedge 41 along a slope inclined at a certain angle, and a spindle 43 is meshed with the second adjustment wedge 42, with both ends thereof being screwed into and supported by the roll stand 30.
In the gap control device 40, the second adjustment wedge 42 meshed with the spindle 43 moves back and forth in a horizontal direction following the direction in which the spindle 43 rotates. The first adjustment wedge 41 which is in surface contact with the second adjustment wedge 42 along the slope of a certain angle moves upwards and downwards depending on the horizontal position of the second adjustment wedge 42. In this fashion, the gap G between the upper and lower dies 11 and 12 is controlled, whereby the outer diameter of the pipe which is to be machined can be controlled.
FIG. 3 is a front elevation view showing the typical Pilgering apparatus.
Referring to FIG. 3, shafts 11a and 12a serving as drive shafts are axially provided in the upper die 11 and the lower die 12, respectively. The shafts 11a and 12a are supported by bearing blocks 31a, 31b, 32a and 32b such that the shafts 11a and 12a are freely rotatable.
The bearing blocks 31a, 31b, 32a and 32b consist of the pair of upper bearing blocks 31a and 31b and the pair of lower bearing blocks 31a and 31b. The upper bearing blocks 31a and 31b are provided on the roll stand 30 such that the upper bearing blocks 31a and 31b are movable with respect to the lower bearing blocks 32a and 32b. The gap control device 40 is disposed on the upper bearing blocks 31a and 31b and supported on the top end of the roll stand 30.
In the Pilgering apparatus of the related art, the gaps of the pair of upper bearing blocks 31a and 31b supporting the upper die 11 can be controlled by manipulating the gap control device 40 such that the gaps of the right and left bearing blocks are the same. There is a problem in that the gaps of the upper bearing blocks 31a and 31b cannot be controlled to be different.
In a specific example of the Pilgering apparatus which performs a Pilgering operation, the die on the ball stand is replaced with a die having a different size according to the size of pipes to be fabricated. When the replacement die is mounted, it is required to adjust the heights of the upper bearing blocks 31a and 31b to different values due to differing assembly tolerances.
However, the related-art gap control device 40 provided on the Pilgering apparatus can adjust the gap only within the range in which the heights of the upper bearing blocks 31a and 31b are the same. When differing assembly tolerances occur during the replacement, it is impossible to accurately align the die shafts.
The information disclosed in the Background of the Invention section is only for the enhancement of understanding of the background of the invention, and should not be taken as an acknowledgment or as any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.